Reinforce the Pond Floor
If the leakage is due to the porous soil the pond was dug into, then there is a way to fill those pores and slow, if not stop, the seepage entirely. However this fix may be costly. First the pond should be drained if possible and filled with sand, clay and other thick, heavy materials which will retain water along with the natural soil. If you want even more of a guarantee of this working, lay down a plastic mesh waterproof lining before laying three to six inches of heavier soil on top, ensuring that the mesh is not punctured. This lining, with the natural absorption of the soil on top of it, will reduce any leak significantly, if not stop it altogether.
Removing trees and shrubs from the edge of the pond itself will eliminate the chance of roots working their way through the farm pond's earthen walls in search of water and causing a leak. However, it might be a good idea to put down some topsoil and fertilizer on the outside of the pond so grass can grow and help hold the walls of the pond together. A limited amount of plant life--like grasses--will hold the soil together as well and keep it thick but not have roots that will go deep enough to damage the structural integrity of the farm pond's walls. If nothing is growing around the pond, the pond wall may dry out and collapse.
Get Rid of Burrowing Animals
If you notice moles, groundhogs, beavers or nutria in your farm pond, it is important to get rid of them if you think they might cause a leak. These animals don't know that your man-made pond can't necessarily hold up to all their activities. These animals can be trapped alive and moved to another location or they can be killed in traps, deterring other animals of their kind from setting up camp there. Crayfish are also a problem in farm ponds, as they like to burrow down under the bottom of the pond, and can burrow through any waterproof lining which will cause a costly leak, as the pond will have to be drained and that section of liner replaced. Traps for crayfish can be found at local bait shops.